The mechanisms controlling the formation and maintenance of neuronal trees are poorly understood. We examined the dynamic development of two arborized mechanoreceptor neurons (PVDs) required for reception of strong mechanical stimuli in Caenorhabditis elegans. The PVDs elaborated dendritic trees comprising structural units we call "menorahs." We studied how the number, structure, and function of menorahs were maintained. EFF-1, an essential protein mediating cell fusion, acted autonomously in the PVDs to trim developing menorahs. eff-1 mutants displayed hyperbranched, disorganized menorahs. Overexpression of EFF-1 in the PVD reduced branching. Neuronal pruning appeared to involve EFF-1-dependent branch retraction and neurite-neurite autofusion. Thus, EFF-1 activities may act as a quality control mechanism during the sculpting of dendritic trees.Morphologies of dendritic trees vary from one neuronal type to another, and the pattern of these arbors determines the manner in which a neuron processes its synaptic or sensory input. However, little is known regarding the mechanisms controlling the outgrowth and maintenance of dendritic trees (1-4). Two mechano-receptors in Caenorhabditis elegans (PVDR and PVDL; right and left, respectively) are responsible for an avoidance response triggered by strong mechanical stimuli to the body (5). The complete neural system of C. elegans has been considered to comprise only simple-patterned neurons (6). However, recent studies show that the PVDs have a more complex morphology (7,8).Here, we established a genetic system to dissect the mechanisms of branch generation and plasticity of arborized neurons in C. elegans. To determine branching patterns, we imaged † To whom correspondence should be addressed. podbilew@tx.technion.ac.il (B.P.), ishemer@email.unc.edu (G.S.). * Present address: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. Note added in proof:After we submitted this report, showed that axotomized PLM sensory neurons fail to reconnect in eff-1 mutants. (table S1). The PVDs contained repetitive structural units reminiscent of multibranched candelabras or menorahs (Fig. 1A). Although the number of menorah branches varied, the menorahs appeared to develop in a stepwise manner from the L2 larva to the adult ( fig. S1). The stereotypical menorah structure is likely to form a functional unit necessary for the PVDs mechanosensory activities.Mutations in the cell fusion gene eff-1 (9,10) affected the pattern of PVDs arborization, resulting in disorganized and hyperbranched phenotypes (Fig. 1B). Moreover, eff-1(ok1021) mutant animals showed reduced sensitivity to strong mechanical stimuli (53%, n = 106) (11). To characterize menorah disorganization in eff-1 mutants, we quantified the number of processes at different degrees of the branching order (primary to senary branches; Fig. 1C and fig. S2). The frequency of secondary and tertiary branching was doubled in the eff-1(hy21) mutant compared with wild type. The eff-1(hy21)...

Whether and how neurons that are present in both sexes of the same species can differentiate in a sexually dimorphic manner is not well understood. A comparison of the connectomes of the Caenorhabditis elegans hermaphrodite and male nervous systems reveals the existence of sexually dimorphic synaptic connections between neurons present in both sexes. Here, we demonstrate sex-specific functions of these sex-shared neurons and show that many neurons initially form synapses in a hybrid manner in both the male and hermaphrodite pattern before sexual maturation. Sex-specific synapse pruning then results in the sex-specific maintenance of subsets of the connections. Reversal of the sexual identity of either the pre- or postsynaptic neuron alone transforms the patterns of synaptic connectivity to that of the opposite sex. A dimorphically expressed and phylogenetically conserved transcription factor is both necessary and sufficient to determine sex-specific connectivity patterns. Our studies reveal new insights into sex-specific circuit development.

SUMMARY Functional and anatomical sexual dimorphisms in the brain are either the result of cells that are generated only in one sex, or a manifestation of sex-specific differentiation of neurons present in both sexes. The PHC neurons of the nematode C. elegans differentiate in a strikingly sex-specific manner. While in hermaphrodites the PHC neurons display a canonical pattern of synaptic connectivity similar to that of other sensory neurons, PHC differentiates into a densely connected hub sensory/interneuron in males, integrating a large number of male-specific synaptic inputs and conveying them to both male-specific and sex-shared circuitry. We show that the differentiation into such a hub neuron involves the sex-specific scaling of several components of the synaptic vesicle machinery, including the vesicular glutamate transporter eat-4/VGLUT, induction of neuropeptide expression, changes in axonal projection morphology and a switch in neuronal function. We demonstrate that these molecular and anatomical remodeling events are controlled cell-autonomously by the phylogenetically conserved Doublesex homolog dmd-3, which is both required and sufficient for sex-specific PHC differentiation. Cellular specificity of dmd-3 action is ensured by its collaboration with non-sex specific terminal selector-type transcription factors whereas sex-specificity of dmd-3 action is ensured by the hermaphrodite-specific, transcriptional master regulator of hermaphroditic cell identity, tra-1, which represses transcription of dmd-3 in hermaphrodite PHC. Taken together, our studies provide mechanistic insights into how neurons are specified in a sexually dimorphic manner.